This invention relates to an outboard motor and more particularly to an improved exhaust arrangement for outboard motors having four-cycle internal combustion engines.
Because of the compact nature of an outboard motor, it has been generally the practice to employ an exhaust system that includes an expansion chamber in the drive shaft housing and a below the water high speed exhaust gas discharge from which the gases from the expansion chamber are discharged to the atmosphere. The expansion chamber performs the function of assisting and silencing of the exhaust gases as does the underwater exhaust gas discharge.
With this type of arrangement, however, it is also the practice to provide an above the water exhaust gas discharge for discharging the exhaust gases directly to the atmosphere and not through the body of water in which the watercraft is operating under at least some running conditions. The reason for the necessity for the above the water exhaust gas discharge is that when the engine is running is low speeds and the associated watercraft is not planing, the underwater exhaust gas discharge is quite deeply submerged. This coupled with the low pressure of the exhaust gases under these running conditions makes the utilization of the underwater exhaust gas discharge impractical if not impossible.
Various arrangements have been proposed for delivering the exhaust gases to the above the water exhaust gas discharge. Generally, these arrangements draw the exhaust gases off from the expansion chamber and pass through a series of orifices or a convoluted path before delivery to the above-the-water exhaust gas discharge. This is done so as to provide not only silencing but to limit the amount of exhaust gases that flow through this path when traveling at high speeds.
These theories are quite practical and have been found efficient in use. However, there is recently a renewed interest in employing four-cycle engines as the power plants in outboard motors. A four-cycle engine, unlike a two-cycle engine, requires a source of lubricant to hold the lubricant that has recirculated through the engine for lubricating purposes. In order to provide adequate storage capacity and to maintain a low profile for the outboard motor, the oil tank is generally positioned in the drive shaft housing.
This positioning for the oil tank exposes it to the exhaust gases and thus, presents a problem of heating the oil tank. If the exhaust gases for the above the water discharge pass in proximity to the oil tank, as is the normal practice, then the oil may become heated even when the engine is running at low speeds. This is undesirable.
It is, therefore, a principal object of this invention to provide an improved exhaust system for an outboard motor.
It is a further object of this invention to provide an improved exhaust system for an outboard motor and particularly one having a four-cycle engine wherein the idle exhaust gases are maintained in an isolated manner from the oil tank.
It is a further object of this invention to provide an improved idle exhaust discharge for a four-cycle outboard motor.